The invention relates to a rotary drill bit for deephole drilling in subsurface earth formations, and in particular to a drill bit including a bit body which is suitable to be coupled to the lower end of a drill string and carries a plurality of cutting elements.
Bits of this type are known and disclosed, for example, in U.S. Pat. Nos. 4,098,362 and 4,244,432. The cutting elements of the bits disclosed in these patents are preformed cutters in the form of cylinders that are secured to the bit body either by mounting the elements in recesses in the body or by brazing or soldering each element to a pin which is fitted into a recess in the bit body. Impacts exerted to the cutting elements during drilling are severe and in order to avoid undue stresses in the elements, the frontal surface of each element is generally oriented at a negative top rake angle between zero and twenty degrees.
The cutting elements usually comprise a front layer consisting of synthetic diamonds or cubic boron nitride particles that are bonded together to a compact polycrystalline mass. The front layer of each cutting element may be backed by a cemented tungsten carbide substratum to take the thrust imposed on the front layer during drilling. Preformed cutting elements of this type are disclosed in U.S. Pat. No. 4,194,790 and in European Patent No. 0029187 and they are often indicated as composite compact cutters, or--in case the abrasive particles are diamonds--as polycrystalline diamond compacts (PDC's).
A general problem encountered with conventional drill bits of the above type is that the degree of bit wear cannot be monitored in an accurate manner. Hence, it may sometimes happen that a hardly worn bit is retrieved to the surface for replacement. Furthermore, it may happen that during drilling in particular formations excessive bit wear takes place while during drilling in other formations hardly any bit wear takes place. Thus, there is a need to enable operating personnel to select optimum operating conditions for particular formations in order to avoid excessive wear rates and to determine an optimum combination between performance and lifetime of rotary drill bits.